1. Field of the Invention
This invention relates to the field of integrated circuit memories. More particularly, this invention relates to write assist mechanisms for use within integrated circuit memories to allow data values to be more readily written.
2. Description of the Prior Art
It is known to provide integrated circuit memories with write assist mechanisms to enable data values to be more readily written to the memory cells. One known system uses two power supply rails connected by respective gates to each memory cell to be written. A first of these power supplies is used to power the memory cell in modes other than a write mode and utilises a relatively high supply voltage level. The second power supply has a lower voltage level and is used during write operations to power the memory cell. The lower supply voltage used during such write operations enables the memory cell to be more readily written. A disadvantage with this approach is that two separate power rails need to be routed through the memory, which uses a disadvantageous amount of space. Furthermore, the switching between these two power supplies is relatively slow such that it is difficult to rapidly switch from writing a memory cell to reading that same memory cell or a memory cell connected to the same power supply.
Another known approach to providing a write assist mechanism is using the memory cell with a gated power supply from a single source. When operating other than to perform a write, the memory cell is supplied with power through the gate. When it is desired to write to that memory cell, the gate isolates the memory cell from the power supply such that the memory cell is unpowered during the write operation. This makes it easier for a new data value to be written into the memory cell. Memory cells are not normally provided with individually controllable power supplies since this would consume a disadvantageous number of gates and reduce the memory density. Rather, a group of memory cells usually share a power supply line, which may be a virtual power supply line separated by a gate from a permanently powered power supply line in accordance with this technique. However, when multiple memory cells share a power supply in this way, all of these memory cells will be isolated from the power supply and be unpowered whilst one of the memory cells is being written. This will leave the unpowered memory cells vulnerable to data loss. This risk is increased when one considers manufacturing and process variations which can occur rendering individual memory cells particularly vulnerable to data loss when unpowered. This approach suffers from the disadvantage of potentially rendering a memory unreliable.